1. Field of the Invention
The present invention relates to a liquid crystal display device of externally charging type and a hand-writable liquid crystal board set including the device. More specifically, the present invention relates to a hand-writable liquid crystal board set comprising: a liquid crystal display device of externally charging type in which the display or erasing of an image is carried out by varying the state of light scattering in the liquid crystal using electro-optical effects, or the provision or removal of static electric charges; display means to display an image on a board screen; erasing means for erasing the recorded image; and a power source for generating a surface potential on the board screen using the display means.
2. Description of the Related Art
A lot of proposals have already been made relating liquid crystal display devices using the electro-optical effects, such as current effect, electric field effect, or heat effect of liquid crystals. Of these, a liquid crystal display device disclosed in Japanese Patent Application Laid-Open No.(sho)55-96922, comprises a pair of transparent conductive layers, and a liquid crystal-dispersed polymer layer sandwiched therebetween in which micro-capsules hermetically holding liquid crystal therein are scattered. Since this liquid crystal display device effects the display by varying the alignment of liquid crystal by turning on and off the application of electric field between electrodes, it is possible to easily obtain a large-sized display screen with a simple structure. Nevertheless, this display device requires to use polarizing plates thus exhibiting drawbacks that the display image is dark and the field of view is narrow.
U.S. Pat. No. 4,435,047 as well as Japanese-translated PCT Patent Application Laid-Open No.(sho)63-501512, discloses a liquid crystal display device which has a simple structure with no polarizing plate, thereby the above drawbacks are eliminated, and which facilitates the provision of a large-sized display screen. For this purpose the liquid crystal display device includes a pair of transparent electrodes, and a liquid crystal-dispersed polymer layer sandwiched between the electrodes in which liquid crystal is finely dispersed in the polymer matrix. The liquid crystal display device having such a structure, however, cannot be applied to a so called externally charging type liquid crystal display, in which recording and erasing can be effected from the outside surface of the display screen.
As an improvement in this respect, Japanese Patent Application Laid-Open Nos.(sho)61-83521 and (hei)3-43715 disclose a liquid crystal display device comprising a liquid crystal-dispersed polymer layer in which liquid crystal is finely dispersed in the polymer matrix and a conductive layer being coated in layer thereon; and an image display method in which a recording image is formed by providing static charges on the display screen from the outside of the liquid crystal-dispersed polymer layer (of the display screen) and thus changing the phase and/or alignment of the liquid crystal.
However, none of these liquid crystal display apparatus described heretofore would allow the hand-writable or erasing of images on the display screen from the outside of the liquid crystal display device.
A liquid crystal display apparatus disclosed in Japanese Patent Application Laying-Open No.(sho)61-83521, is to perform display and erasing of the record image by providing or removing static charges in a form of corona charge flow from the outside of the display screen of the liquid crystal display device. The present inventors hereof have investigated liquid crystal display apparatuses of this kind, however and have found the fact that the provision of electric charges to the display screen could cause the cholesteric liquid crystal to change the phase thereof to form a recording image, but the removal of the static charges could not bring back the phase-changed cholesteric liquid crystal to the initial state, or could not erase the recorded image. In order to erase the recorded image, it was found that the liquid crystal must be heated above the phase transition temperature, or be further provided with an electrically conductive sheet through which an a.c. electric field can be applied. It was also found that, for repetitions of display, the remaining static charges must be removed to effect next display. In sum, it was found that complicated operations were required to display and erase recorded images repeatedly.
In a liquid crystal display device disclosed in Japanese Patent Application Laid-Open No.(hei)3-43715, if electric charges are provided to the display screen from the outside thereof by any of various charging techniques, the liquid crystal is aligned and this changes the state of light scattering in the liquid crystal display device to display a recorded image. This disclosure teaches that, in order to erase the recorded image thus obtained, it is necessary to provide opposite electric charges equivalent to the amount of the static charges present only to the area of the recorded image. In practice, however, it is very difficult to perform such an operation since the amount of the opposite charges may be slightly more or less than that of the static charges. This situation makes the erasing operation uneasy. Moreover, the apparatus of the disclosure disadvantageously exhibits shortness of memory retention of the recorded image in a 20.degree. C. air at a relative humidity of 90% or under environment of high humidity.
Some hand-writable liquid crystal board sets have been tried to be manufactured by utilizing the above described liquid crystal display apparatuses, but the reasons described above disturbs the achievement. In other words, the former apparatus needs complicated display and erasing processes to repeatedly display images and erase them, or is problematic in its handling and performances. For the latter apparatus, erasing of the recorded image is unfeasible, and moreover, the apparatus is problematic in its durability of the liquid crystal board and in its performance of memory retention of the recorded images under a high humidity environment. For these reasons, it has been impossible to obtain a hand-writable liquid crystal board having a good operativity and sufficient durability and performances thereof.